A trench power metal oxide semiconductor device is a common type of semiconductor device. FIG. 1 is a diagram illustrating the cross-section of a typical power MOSFET device. In this example, device 100 includes a source 104 made of N+-type material, a body 106 made of P-type material and a drain 108 made of N-type material. Device 100 also includes a gate 102 that is recessed from the top surface of source 104 and body 106. The recessed gate is typically a result of the fabrication steps used to produce the transistor.
While this type of MOSFET device with recessed gate has proven useful, several problems remain. One of the problems associated with the current device structure is the on state resistance. Since the bottom portion of source region 104 is typically required to overlap with recessed gate 102 to insure proper device operation, the depth of source region 104 typically needs to meet a certain minimum. The current, which flows through the source region at a minimal required depth, leads to an on state resistance having a minimum value that is not easily reduced.
Another problem associated with the typical source depth requirement is gate capacitance. Since the channel typically requires a minimal channel length, a deeper source means that a deeper trench is typically required to accommodate the gate, thus increasing the gate capacitance and lowering the switching speed. The lateral diffusion of a deeper source typically requires a larger contact. As such, the reduction in device size and the increase in cell density are both limited.
Another problem associated with the current design is that the parasitic bipolar NPN transistor formed by source 104, body 106 and drain 108 is often easily turned on, thus limiting the operable range of the device. FIG. 2 is a diagram illustrating a circuit model of a MOSFET device similar to the one shown in FIG. 1. Bipolar transistor 202 is a parasitic transistor formed between the source and the drain. With a thick source region, the distance between the body contact and the channel region is high, so is the resistance between the base-emitter terminals (resistor 204). As a result, a small amount of leaked current flowing through the bipolar transistor may lead to a voltage drop between the base and the emitter that exceeds the threshold required for turning on the bipolar transistor.
It would be desirable to develop a MOSFET device with shallower source to improve cell density and to reduce on state resistance. It would also be useful if the resistance between the base and emitter of the parasitic bipolar transistor could be reduced so that the parasitic bipolar transistor would not be turned on easily. It would also be useful to develop a power MOSFET device with shallower trench depth and smaller gate capacitance to improve the device switching speed.